This invention relates to an improvement of method and apparatus for varying the cross-section of the beam of electrically charged particles and projecting the so-varied beam of electrically charged particles onto a target, which method and apparatus is particularly useful in making semiconductor devices, integrated circuits and other microscopic machinework. Electron beam exposure apparatuses have been used in producing a variety of semiconductor devices and master masks, and there has been an ever increasing demand for raising the exposure speed in the hope of improving the productivity. Such a demand, however, cannot be satisfied with recourse to the electron beam sweeping-and-projecting method according to which a beam spot of very small and definite diameter sweeps and blots one part after another on a target. In the hope of meeting such a demand there has been recently proposed an electron beam varying-and-projecting method according to which the cross-section of the electron beam is varied both in size and shape, and the so-varied electron beam is allowed to pass to a target.
Referring to FIG. 1, the principle of the electron beam varying-and-projecting method is described. As shown in the drawing, an electron beam projecting apparatus uses two apertured masks M.sub.1 and M.sub.2 each having a rectangular or square aperture H.sub.1 or H.sub.2, and two deflecting means D.sub.1 and D.sub.2 capable of deflecting the electron beam transverse to the beam path in all directions, thus causing the cross-section of the electron beam to vary both in shape and size before reaching a selected part of the target W.
In operation, the first apertured mask M.sub.1 is illuminated with the electron beam. After the electron beam passes through the square aperture H.sub.1 of the first mask M.sub.1 and through the lens L.sub.1, it is deflected by the deflecting means D.sub.1 with respect to the square aperture H.sub.2 of the second apertured mask M.sub.2. Thus, the clear image of the square aperture H.sub.1 of the first mask M.sub.1 appears on the second apertured mask M.sub.2 in the overlapping relation with the aperture H.sub.2 of the second mask M.sub.2. After the electron beam passes through the second mask M.sub.2 and through the second lens L.sub.2, it is deflected by the deflecting means D.sub.2 so that the electron beam whose cross-section corresponds to the overlapping area of the apertures H.sub.1 and H.sub.2 of the first and second masks M.sub.1 and M.sub.2 strikes against a selected part of the target W.
As is readily understood, by controlling the transverse deflections of the beam at two different positions a variety of electron beams as shown in FIG. 2A (shaped area) result. In producing for instance semiconductor devices, a variety of patches are projected onto a semiconductor substrate one after another to form a desired pattern thereon in the form of patchwork. Apparently the electron beam varying-and-projecting method is most effective to reduce the exposure time involved for forming a given pattern on the target, requiring a short exposure time compared with blotting with the electron beam spot, and accordingly raising the productivity of semiconductor devices.
The electron beam varying-and-projecting method is satisfactory for almost all purposes. This method, when using an increased beam current, however, is difficult to use in microscopic machinework requiring resolution as high as 0.1 micrometers. Because the polygonal image of the overlapping area of the first and second apertures appearing on the target is not clear at those sides which are defined and are in contour with corresponding sides of the first aperture M.sub.1. The inventors found that the blurring of the image on the target is mainly caused by the Coulomb's repulsion between electrons or space charge effect while travelling from the first apertured mask to the target, as is later described in detail. Also, the inventors found that the thermal deformation of the second aperture M.sub.2 appreciably contributes to the distortion of the image of the overlapping space of the first and second apertures. The thermal deformation is caused by selectively preventing a large part of the electron beam from passing through the second aperture and by allowing such a large part of the electron beam to dissipate in the second apertured mask. For these reasons the precision of the final image of the overlapping space of the first and second apertures is lowered. Still disadvantageously, the heating of the second mask shortens the effective life of the second mask.
The object of this invention is to provide an improvement of the electron beam varying-and-projecting method and apparatus which is free from the defects above mentioned, thus assuring that a clear and precise image of the overlapping space of the apertures of the masks is formed on the target, still retaining an increased exposure rate. The image sharpness or resolution attained by this invention makes the electron beam varying-and-projecting art increasingly useful in microscopic machine-work as required in producing minute semiconductor devices. To attain the object of this invention a method of varying-and-projecting a beam of electrically charged particles comprising the steps of: providing at least three apertured masks; illuminating the aperture of the uppermost mask with the beam; deflecting the beam between some of the masks upstream so as to reduce the beam to the amount which is required in forming a desired image on a target; and deflecting the so-reduced beam between masks downstream so as to trim the cross-section of the beam into the exact shape of the desired image.
In another aspect of the method of varying-and-projecting a beam of charged particles according to this invention the beam is deflected between some apertured masks upstream so as to allow the beam to have a marginal portion to the exact shape of the desired image, and the so-modified beam is deflected between the apertured masks downstream so as to remove the marginal portion from the beam.
An apparatus for varying-and-projecting a beam of electrically charged particles according to this invention comprises: a gun for supplying the beam; at least three apertured masks; deflectors and lens means; and means for supplying electric signals of two kinds to said deflectors, deflectors associated with some of the apertured masks upstream being responsive to the electric signal of one kind for reducing the beam to the amount which is required in forming a desired image on a target, whereas deflectors associated with the apertured masks downstream being responsive to the electric signal of the other kind for trimming the beam into the exact shape of the desired image. In another aspect of the apparatus for varying-and-projecting a beam of electrically charged particles according to this invention some of said deflectors and associated lens means are arranged coextensive with each other.